The present invention relates to a speckle noise discrimination method and an ultrasonic tomography apparatus including a judging and removing circuit of speckle noises having small bright points appearing at random due to interference of scattered waves generated at various phases by sufficiently smaller reflection bodies of tissues of a living body than a wavelength of an ultrasonic wave in the ultrasonic tomography apparatus which utilizes an ultrasonic wave to measure a tomographic image of any part in an object to be examined.
Generally, noises named speckle noises and having an irregular period are mixed into an ultrasonic image obtained by the ultrasonic tomography apparatus. It is considered that the speckle noises are small bright points appearing at random due to interference of scattered waves generated at various phases by sufficiently smaller reflection bodies of tissues of a living body than a wavelength of an ultrasonic wave. It is known that the speckle noises appear to be superposed on an original reflected echo signal over the whole area of the image of the brightness modulation mode. More particularly, for example, in an ultrasonic image of the heart, the speckle noises are superposed on a reflected echo signal of a structure such as a wall of the heart, so that the original signal is disturbed to deteriorate the image quality such as flicker of an image and the speckle noises also appear in an image of a part having no reflective body such as the true lumen so that the image quality thereof is deteriorated. In general, it is known that the pattern of the speckle noises is largely changed depending on the arrangement of minute scattered bodies distributed in a living body and the pattern of the speckle noises is also changed when tissues of the living body are moved. However, a degree, a direction and brightness of the change of the pattern are not uniform. It is confirmed that when a living body is being moved, the speckle noises are changed irregularly.
In view of the above circumstances, a conventional ultrasonic tomography apparatus uses a frame correlation processing circuit as shown in FIG. 10 to perform the adding and averaging process with secular weight, so that the speckle noises in a moving image are reduced. The frame correlation processing circuit serves to reduce the speckle noises by means of a method generally named the "frame correlation" or the "scan correlation". The frame correlation processing circuit includes, as shown in FIG. 10, an operation processing circuit 1 constituted by, for example, a weighted adder, a frame memory 2 having the function of recording data of an image of the brightness modulation mode, and a control circuit 3 for successively generating addresses of the frame memory 2 in synchronism with data of an ultrasonic image inputted to an input terminal D of the frame memory 2.
In the frame correlation processing circuit, the frame memory 2 outputs data recorded in the address selected by the control circuit 3, that is, data before one frame from an output terminal E. The output data from the output terminal E is supplied to an input terminal B of the operation processing circuit 1. Then, the operation processing circuit 1 correlates (adds and averages) data at the current time sent from an ultrasonic body unit not shown and supplied to an input terminal A with the data before one frame inputted to the input terminal B and produces the correlated result from an output terminal C. Thus, the output data from the output terminal C is sent to the input terminal D of the frame memory 2 as data at a new tense. After the data at the new tense is inputted to the frame memory 2, the control circuit 3 directs the frame memory to record the data in the same address as that of the last time.
The correlation process performed by the operation processing circuit 1 is now described. When a numerical value inputted to the input terminal A of the operation processing circuit 1 is expressed by a, a numerical value inputted to the input terminal B is expressed by b, and the operation result produced in the output terminal C is c, the weighted addition and average processing is, in most cases, performed by the correlation coefficient of X and Y having the following relation: EQU a.multidot.X+b.multidot.Y=c (1)
where X&lt;1 and Y&lt;1 and there is the following relation: EQU x+Y=1 (2)
Generally, the correlation coefficient of X and Y is selected depending on a diagnostic part of the object to be examined and the magnitude of noises.